Imbibition processes in fractured chalk core plugs with connate water mobilization

A displacement process in a chalk core plug with an induced axial fracture uses the fluid system H₂O-D₂O-n-decane. Initially the plug contains a mixture of D₂O and n-decane with oil saturation equal to 88 %. The plug is flooded with H₂O. The displacement process is monitored by NMR with a chemical shift technique enabling spatial resolution of the H₂O and oil signals. The D₂O component is not directly detected, but is calculated from the difference between the actual signal intensities and the signal intensities in a situation where the pore space is completely saturated with H₂O and oil. 

H₂O-D₂O-oil constitutes a partly miscible fluid system which is an analogy to the injection water-formation water-oil system of a hydrocarbon reservoir under waterflooding. 

The injected H₂O traverses the sample as a well-defined front together with capillary uptake from the fracture plane. Ahead of the H₂O front the D₂O of the sample is mobilized and travels as a bank in front of the H₂O. Though the initial concentration of D₂O is only 12 % the D₂O concentration reaches 50 % at the bank. 

The experiment was simulated by means of ECLIPSE with a fair match of the displacement process. 

The injected H₂O has only made limited contact with the produced oil of the sample. In case of chemical EOR processes connected to waterflooding of hydrocarbon reservoirs a similar displacement situation could seriously affect the efficiency of the EOR process